The purpose of this study is to determine the role of cytoskeletal and viral proteins in the maturation of a new group of endogenous type B viruses persistent in murine, hamster and marsupial cells. Intracytoplasmic type A particles known to be precursors to such viruses are closely associated with microtubule organizing centers at centrioles in interphase cells and at kinetochores in metaphase-arrested cells. The intracytoplasmic transport of the particles to the cell surface and their release at the cell membrane appears to be dependent on the intact microtubule system of the cell. These results were obtained by studying the intracellular distribution and the mechanisms of intracellular movement of type A particles with a) immunoelectron microscopy (EM) of cytoskeletal proteins, b) quantitative EM of such particles at isolated chromosomes suggesting the transfer of these particles from the cytoplasm to centromeric regions during early metaphase in conjunction with microtubule assembly and c) by employing microtubule inhibitors and correlating morphologic with biochemical events, direct evidence for a major role of the cytoskeleton (microtubules) in the transport and release of type A particles of the cell surface was demonstrated.